1. Field of the Invention
The present invention relates to a waterproof optical fiber cable having incorporated therein a filler, namely a water blocking material, for preventing water from penetrating into the cable from outside.
2. Description of the Prior Art
When the sheath of an optical fiber cable ruptures locally, water naturally ingresses into the cable to impair the light transmission characteristics of the cable.
A system has been proposed for optical fiber cables for early detection of a break in the cable sheath and therefore ingress of water into the cable by monitoring the pressure of a gas filled in the interior of the cable to a high pressure. However, the proposed monitoring system is costly and requires expensive cable systems.
It has also been proposed to provide a water blocking layer beneath the cable sheath and fill the inside space of the layer with a water blocking material in order to directly prevent water from entering the interior of the cable even when a break occurs in the cable sheath, for instance, in JECE-JAPAN-NCR (The Institute of Electronics and Communication Engineers of Japan, National Convention Record) No. 1901 (Page 7-344) 1981, JECE-JAPAN-NCR No. 366 (Page 2-102), No. 1810 (Page 7-252) and No. 1811 (Page 7-253), 1982. The proposal has the advantage of being economical because the above monitoring system is made unnecessary. Water blocking materials known for use in optical fibers are solid or a highly viscous liquid at room temperature. Accordingly such a material is heated to melt or decrease its viscosity before being filled into the cable during the cable making process. The conventional water blocking material has the following drawbacks because the material invariably solidifies or becomes highly viscous while contracting when cooled after filling.
(1) Owing to contraction, a clearance occurs at the interface between the water blocking layer and the water blocking material or at the interface between the water blocking material and the optical fiber in the cable core, with the result that water, if entering the cable, runs through the clearance longitudinally of the cable.
(2) The optical fiber, which is thin, flexible and therefore easily bendable, is restrained by the water blocking material which rapidly becomes viscous or consistent when cooled after filling. Moreover, the contraction of the material causes microbending of the fiber and results in an increased light transmission loss. Especially when the cable is used during winter or in a cold climate, the material undergoes more marked contraction and produces a greater restraint to entail a further increased light transmission loss.
(3) Because the water blocking material is difficult to remove from the cable after solidification, it is difficult or requires a long period of time to make high precision cable connections.